If the US and other nations don't protect Ukraine now it will be impossible to convince any nation in the future not to arm themselves with nuclear weapons.
If Ukraine manages to fight off the Russians without our aid they will probably set about developing nuclear weapons.
And then we'll have another nuclear armed nation - one with some of the most modern drones and a large army with combat experience.
It is in the interest of our nation to have them on our side.
For the States, united, to make a binding promise to a foreign country requires consent from the states in the form of 2/3 of their delegates (the Senators) to ratify something called a treaty.
In the case of Ukraine there is a piece of paper signed by Bill Clinton, which has no binding effect on the 50 states which would have to provide the men and the money to fight a war.